1. Field of the Invention
This invention relates to a rotary disc-type apparatus for biological treatment of sewage.
2. Description of the Prior Art
A typical sewage treatment apparatus of the rotary disc type comprises a rotary shaft and a plurality of rotary plates secured to the rotary shaft in an axially spaced-apart relationship, said rotary plates being adapted to be rotated while they are partly dipped in sewage within a treating tank. As a result of this rotation, the rotary plates are exposed alternately to the sewage and to oxygen in the atmosphere whereby aerobic bacteria adhering to the rotary plates grow. The bacteria biosorb, coagulate and oxidize organic components in the sewage thereby to purify the sewage. A rather large force is required to rotate the rotary plates having such aerobic microorganisms adhering thereto while dipping them partly in the sewage, and in order to withstand such a force, the rotary plate assembly should have sufficiently high strength.
In one type of known sewage treatment apparatuses including rotary discs, plastic rotary plates are arranged in disc form respectively about plastic mounting plates securable to the periphery of a rotary shaft and bonded by fusion to the mounting plates, and then the mounting plates are fixedly mounted around the rotary shaft. A method is also known which involves bonding the rotary plates to the rotary shaft by packing sponge-like plastics in the space between them (Japanese Patent Publication No. 21819/1977).
In these known apparatuses, however, the strength of the joint between the rotary shaft and the rotary plate assembly which are bonded together through the mounting plates or sponge-like plastics is not sufficiently high so that the joint portion often undergoes damage.
The general trend of the art is to increase the diameter of the rotary plates in an attempt to increase the sewage treating ability of the apparatus without an appreciable increase in installation space. A rotary plate having an increased diameter is often built by assembling a plurality of fan-shaped plates. In the aforesaid conventional apparatuses, a complex operation is required to fix a rotary plate consisting of a plurality of such fan-shaped plates accurately in a predetermined position. Furthermore, bonding of the assembly by fusion requires an additional device designed for this purpose. For these reasons, it is difficult according to the prior techniques to build a sewage treatment apparatus at the site of its use. On the other hand, when a sewage treatment apparatus built in a factory is to be transported to the site of use, there is a restriction on its size.